Do you mean peer IDs? Those are short strings of random characters (not a number).
The signalling server only guarantees that peer IDs are unique for everyone currently connected to the server. An ID may well be re-used for someone else in future after they disconnect from the signalling server. As actual data connections are peer-to-peer and those can run without a connection to the signalling server, it's also possible there are two active peers with the same peer ID. The likelihood of that is based on the length of IDs as that is how many possible combinations there are; currently the signalling server uses four characters which isn't that many (1-2 million combinations, so not enough to rule out collisions).